How Fluid Moves Between Blood and Tissues

Capillaries let gases and other small substances move between blood and tissues, but they also allow a small amount of fluid to leave the bloodstream. Most of that filtered fluid returns through lymphatic vessels rather than being routinely pulled back into the venous end of the same capillary. This balance helps keep tissue fluid under control.

Gases and small dissolved substances move by diffusion Capillaries are the smallest blood vessels. Their thin walls bring blood close to tissue cells, allowing substances to move between the blood and the fluid around those cells. Oxygen and carbon dioxide move mainly by diffusion , from higher concentration to lower concentration, down what is called a concentration gradient. Many other small dissolved substances also cross the capillary wall. Water behaves differently. When water moves in bulk as fluid across the capillary barrier, the movement depends mainly on pressure and the properties of the barrier, not simply on a concentration gradient. A small amount of fluid normally filters into tissues Blood pressure inside capillaries tends to push water out towards the tissues. Plasma proteins, especially albumin, create an opposing protein related pull called oncotic pressure. A thin layer called the endothelial glycocalyx lines the inside of the capillary and forms an important part of the filtering barrier. It helps control how water and proteins cross the vessel wall. Older diagrams often show fluid leaving near the arterial end of a capillary and most of it being drawn back in near the venous end. In most continuous capillaries, the common type found in many tissues, that picture is too simple under stable conditions. A small net amount of fluid usually continues to filter out along much of the capillary. Sustained reabsorption, meaning fluid moving back into the capillary, is limited at the venous end. Brief reabsorption can occur when pressures change, and some specialised tissues behave differently. Lymphatic vessels return the filtered fluid The fluid around tissue cells is called interstitial fluid . Lymphatic capillaries collect much of the fluid and proteins that have left the blood vessels. Once this fluid enters a lymphatic vessel, it is called lymph . It travels through the lymphatic network and eventually returns to the venous circulation. This means the lymphatic system is part of normal tissue fluid balance. It is not simply a backup system for fluid that capillaries failed to reabsorb. Oedema develops when fluid builds up faster than it can be cleared Oedema means excess fluid has accumulated in the tissues. This can happen when capillary pressure rises, plasma protein levels fall enough to weaken the opposing oncotic effect, the vessel wall becomes more permeable, meaning fluid and proteins can cross it more easily, or lymphatic drainage is impaired. Salt and water retention can also worsen swelling by increasing the amount of fluid in the circulation and raising venous and capillary pressures. More than one mechanism can act at the same time.

Most continuous capillaries have a small net outward filtration under stable conditions. Lymphatic vessels, rather than routine venous end reabsorption, provide the main route that returns this filtered fluid and escaped proteins to the circulation.